There are two general types of control systems for conventional ventilators. A first type is delivery of gas to a patient based on a frequency selected by the clinician. The frequency selected delivery is independent of patient activity. This control system is used when the patient is non-alert, sedated, unresponsive or paralyzed. In this type of system the ventilator is breathing for the patient. A second type of control system is delivery of gas to the patient in response to an inspiratory effort created by the patient. This type of ventilation helps the patient breathe. There are also ventilators and modes of ventilation that combine the two types of control systems.
In the case of a control system that responds to patient breathing effort, breath effort sensors are required to detect inspiration. In basic conventional systems, the breath sensors detect the start of inspiration using a pressure or flow sensor. The inspiratory effort sensor is located somewhere in the path of ventilation gas delivered by a ventilation gas delivery circuit. A ventilation gas delivery circuit is generally defined as the path of respiration gas delivered by a ventilator. The inspiratory effort sensor may be either inside the ventilator, or in the tubing between the ventilator and the patient, including at the patient end of the tubing. Various attempts have been made to place the inspiratory effort sensor(s) inside the patient, or externally attached to the patient to improve breath effort detection and/or improve response time of the ventilator gas delivery.
Pressure or flow sensors within the ventilation gas delivery circuit have successfully been used to detect the start of inspiration to trigger the ventilator to deliver gas to the patient. However, when there is a need or desire to measure the entire respiratory curve in addition to start of inspiration, sensors within the ventilation gas delivery circuit produce inadequate results because the gas being delivered by the ventilator also moves past the sensor. Thus, the sensor no longer measures the patient's respiration, but rather the gas delivered through the ventilation gas circuit. In a closed ventilation system, the ventilator activity approximates the overall lung activity, hence this positioning of sensors may be adequate. In an open ventilation system, or in ventilation systems that augment a patient's spontaneous breathing, sensors within the ventilation gas delivery circuit are inadequate in measuring the entire respiratory curve.
Sensors not within the ventilator gas delivery circuit have the ability to measure the entire respiration activity. For example, chest impedance sensors can be used to measure the entire respiratory curve of a patient and to use that signal to control the ventilator and synchronize the ventilator to the patient's breathing. Although an improvement, this approach has the disadvantage that the chest impedance signal is prone to drift, noise and artifacts caused by patient motion and abdominal movement. In another technology, neural activity related to the respiratory drive is used to measure the respiration of a patient. However, this has the disadvantage that it is invasive and requires electrodes typically placed in the esophagus to detect the neural activity.
U.S. Non-Provisional patent application Ser. No. 10/870,849 (U.S. Printed Publication 2005/0034721), which is incorporated by reference in its entirety above, describes a new form of breath sensing with sensors not within a ventilation gas delivery circuit. The sensors may be located in the airway of a patient, for example, in the patient's trachea, but not within the ventilation gas delivery circuit. In this manner, the gas delivery from the ventilator may not dominate the sensor measurements. This intra-airway sensor may measure naturally inspired gas flow of the patient, naturally exhaled gas flow of the patient, and the effect of the ventilator gas delivery on lung volumes. The sensor may not measure gas flowing in the ventilator delivery circuit as in conventional systems. This breath sensing method may then measure, not just the start of inspiration, but the entire respiratory pattern of the patient. This may be advantageous to optimize the synchrony of the ventilator to the patient's natural breath pattern, so that the patient is comfortable. Also, if the goal is to provide therapy during different portions of the respiratory curve, such as during the middle of inspiration, or during a particular pan of the expiratory phase, then this method may be used to accurately measure the entire respiratory curve. This new breath sensing technology, however, may not be simple or obvious to reduce to practice. Sensors within the airway of the patient are prone to problems stemming from tissue interaction, patient-to-patient variability, variability within a given patient over time, and a variable physiological environment that can not be controlled. For example, debris in the airway may collect on the sensors and may cause signal artifacts and disrupt the sensors' ability to accurately and reliably measure the entire breath curve. Or, the sensor could come into contact with the tracheal wall, which may disrupt the sensors' signal. Alternatively, tracheal movement during breathing can affect the signal.
Need exists for improved breath sensing systems and methods for ensuring reliable and accurate breath measurements.